1. Field of the Invention
The invention relates to control of a vehicle provided with a powertrain having a driving source and a transmission. More particularly, the invention relates to control of a vehicle, which can realize vehicle behavior which matches that required by a driver.
2. Description of the Related Art
An automatic transmission connected to an engine in a vehicle typically automatically controls the vehicle by determining the speed ratio according to the accelerator operation amount of the driver and the vehicle speed. During normal running, the speed ratio is set on the higher speed side on a shift map as the accelerator operation amount decreases or the vehicle speed increases.
The driving mode (e.g., comfort, economy, sport, etc.) selected by the driver can be identified based on the driving operation of the driver, and that mode can also reflected in the shift control. For example, the shift map is changed according to the driving mode such that the automatic transmission is controlled so that it does not easily upshift but easily downshifts or so that it easily downshifts but does not easily upshift. There are also vehicles that have a running mode selector switch (i.e., a switch that enables the driving mode to be manually switched between comfort mode, sport mode, economy mode, etc.) to detect this kind of driving so that the driver can manually select the driving mode.
Japanese Patent Application Publication No. 2001-132833 (JP-A-2001-132833) describes a shift control apparatus that is provided with a plurality of driving modes such as a sport mode which maintains a high engine speed and engages the engine brake, in addition to a normal running mode used for normal running. This shift control apparatus aims to minimize the number of time constants used in shift control while obtaining the same driving feel irrespective of the driving mode when the same driving operation is performed. This shift control apparatus includes a post-shift speed ratio calculating portion that calculates the speed ratio that will be realized after a shift in a continuously variable transmission according to the driving state of the vehicle, a first shift mode according to normal running, and a second shift mode that increases the engine braking force by restricting, according to the vehicle speed, a lower limit value of the post-shift speed ratio when the accelerator pedal has been released, i.e., is not being depressed. Further, this shift control apparatus is a shift control apparatus of a continuously variable transmission that is provided with a shift mode selecting portion that selects the shift mode from among these shift modes, a time constant calculating portion that calculates a time constant according to the driving state of the vehicle, a calculating portion that calculates a target speed ratio based on this time constant, and a shift control portion that controls the speed ratio such that the actual speed ratio matches this target speed ratio. The time constant calculating portion calculates the time constant according to the difference between the post-shift speed ratio and the target speed ratio or the actual speed ratio, and also has a time constant correcting portion that corrects the time constant so that the shift speed becomes faster when the shift mode selecting portion has selected the second shift mode.
According to this shift control apparatus of a continuously variable transmission, in addition to the first shift mode for normal running, a second shift mode is also provided which restricts the lower limit value of the post-shift speed ratio according to the vehicle speed and thus increases the engine braking force. The first or second shift mode can be selected, and the target speed ratio when the shift is performed is calculated according to the time constant that was set beforehand. This time constant is calculated based on the difference between the post-shift speed ratio and the target speed ratio or the actual speed ratio. However, in the second shift mode, the time constant can be obtained by correcting the time constant of the first shift mode so that the shift speed becomes faster. This obviates the need to provide a time constant map and the like for each driving mode, thereby reducing manufacturing costs. Also, the lower limit value of the post-shift speed ratio is restricted so the shift speed can be made faster according to a driving operation, e.g., the depression amount of the accelerator pedal, in the second shift mode as well which has a smaller shift width than the first shift mode. Accordingly, the same shift speed can be applied to the same driving operation irrespective of the driving mode so an unpleasant driving sensation is not imparted to the driver.
However, according to JP-A-2001-132833, a first running mode which is a normal running mode used for normal running, and a second running mode which is a sport mode that maintains a high input shaft rotation speed NT and utilizes the engine brake are provided. A speed ratio according to the driving state is then controlled based on the driving mode detected by the selector switch that is operated by the driver. Typically, in the second running mode which utilizes the engine brake, the engine speed increases thus increasing the noise and adversely affecting quietness. The driver therefore must choose between the responsiveness of the sporty running and the quietness of the normal running, select the driving mode him or herself, and operate the selector switch accordingly Therefore, the driver must constantly switch between driving modes in order to achieve both the responsiveness of sporty running and the quietness of the normal running.